A peptide preparation of the pineal, epithalamin, when administered chronically, exerts a modulating influence on the diurnal fluctuations of the motoric activity of rats. It also decreases the amplitude of apomorphine stereotypy and induces a restructuring of a modulatory character in its rhythmic structure. It is hypothesized that the pineal peptides participate in the maintenance of homeostasis by means of the stabilization of rhythmic processes of various periods.